Correct option is B
Pulverized fuel (e.g., coal crushed to a fine powder) has a significantly increased surface area compared to lump coal. This allows for rapid and efficient combustion when mixed with air. The burning rate is high due to the large surface area exposed to oxygen, and it is controllable because the fuel-air mixture can be adjusted precisely. This is why pulverized fuel is widely used in modern boilers and furnaces.
Incomplete combustion occurs when there is not enough air (oxygen) to fully burn the fuel. This results in the formation of soot (unburned carbon particles) and carbon monoxide (CO). The soot appears as black smoke in the exhaust. Providing sufficient air is crucial to avoid smoking and ensure complete combustion.
Excess air is primarily supplied to ensure complete combustion (to avoid smoke and CO). However, excess air does not control flue gas temperature; in fact, it lowers the temperature because the extra air (which is at ambient temperature) dilutes the combustion gases and carries away additional heat. This reduces the efficiency of the system.
Sulphur in fuel is undesirable. When burned, it forms sulphur dioxide (SO₂), which can further oxidize to sulphur trioxide (SO₃) and react with water vapor to form sulphuric acid (H₂SO₄).
